Quite right too, it is a brand name. But as far as most etymological theories are concerned, it’s also an acronym. As well as an anacronym. With a reference to an obscure cartoon character thrown in for good measure. Confused? You’re not the only one...

If this theory is correct, then jeep is an example of what is known as a “respelled initialism”, an acronym whose letters have been spelled out phonetically to form a whole new word. Linguistically these “respelled” acronyms—also known as “vocologues”—comprise a fairly rare class of words, but understandably so: after all, acronyms are motivated by brevity, so there’s little point in making them any longer than they need to be. Nevertheless, a handful of examples have emerged over the years, including emcee (from MC, a “master of ceremonies”), deejay (from DJ, a “disc jockey”), the brand name Esso (from SO, “Standard Oil”), and, most familiar of all, okay.

In the case of jeep, of course, the final –ee sound of “GP” (or “gee pee”) isn’t pronounced, which makes itan example of an even rarer class of words known as “clipped initialisms”—namely, acronyms that have been respelled phonetically, then shorted again. Veep, as a nickname for the Vice President, and Beeb, as a nickname for the BBC, both likewise fall into this category, but examples of this particular linguistic phenomenon are unsurprisingly few and far between.

There are a handful of other competing theories of the origin of jeep (at least one of which is outlined here), but most etymologists tend now to sign up to this “GP” explanation. However, many also agree that this particular story doesn’t end there, and that jeep was, somewhere along the line, influenced by something else—something, it’s fair to say, rather unexpected.

On 17 January 1929, Popeye The Sailor Man made his first appearance in print in the Thimble Theatre comic strip. Created by the US cartoonist EC Segar, as the series became increasingly popular more and more characters were introduced to the storyline, including Popeye’s mooching companion Wimpy, his bullying nemesis Bluto, and his stridewallop girlfriend Olive Oyl (who had already made her first appearance in a different comic series ten years earlier). Popeye and Olive eventually adopted a son, Swee’Pea, tracked down Popeye’s estranged father, Poopdeck Pappy, and in 1936 encountered “a mysterious strange animal” called Eugene the Jeep.

Eugene was introduced to the Popeye series when Olive Oyl was given a “jeep”—a highly intelligent dog-like animal with bright yellow fur and a large red nose—as a gift from her uncle. Puzzled by the creature’s appearance, in one edition Popeye calls in an expert, who enthusiastically explains that a “jeep” is “an animal living in a three-dimensional world … but really belonging to a fourth dimensional world.” Throughout several subsequent episodes and escapades, Eugene is ultimately shown being able to travel through time, walk through walls and doors, and teleport effortlessly from one place to another. Put another way, the “jeep” could go wherever he wanted, whenever he wanted to.

Not long after Eugene made his first appearance in 1936, the Willys-Overland Motor Company in Toledo began manufacturing its model MB Army Truck. Powerful and robust, and able to cross practically any terrain, the MB seemed to embody all of Eugene’s most impressive capabilities. As a result, it soon became known as the “jeep”—a nickname partly inspired by the earlier military slang term “GP”, and partly inspired by Popeye’s bizarre teleporting pet.
As a representative of Willys-Overland explained in a letter in 1944:

We feel that the word [jeep] originated with Segar, King Features cartoonist, who until his recent death wrote the Popeye strips. You will recall that in this feature there was a character called “Jeep” which lived on orchids and could go anywhere and do anything. It is our contention that the boys in the service picked this name up from Segar and applied it to the Willys vehicle which has many of the “go-anywhere, do anything” characteristics of the Popeye character.

After the outbreak of the Second World War, the MB became the focus of numerous public demonstrations, all of which helped to popularize the word jeep outside of military slang: as early as February 1941, a publicity stunt was organised in which a Willys truck was driven up the steps of the Capitol Building in Washington DC, with a local newspaper report noting that “the Army’s new scout cars” were already “known as ‘jeeps’”.
Soon, all earlier uses of the word had vanished, and the name jeep had established itself as a standard nickname for any relatively small, yet still relatively powerful, truck.